A female who has at least 2 out of 4 of the following characteristics- sloppy, lousy, annoying, and grose. The fact that sloppy/grose and lousy/annoying are closely related is no mistake and makes the term slag very hard to avoid for slaggish girls. Can also be used to describe males, but this is frowned upon.

"Burt,she was a SLAG!"
"why?"
"She was grose and sloppy and overall lousy."
"yeah....slag."